1. Field of the Invention
This invention relates generally to low pass filters for microwave signals. More particularly, it relates to providing improved frequency characteristics in the microwave spectrum for such filters.
2. Description of Related Art
The microwave portion of the spectrum, usually defined as extending from roughly 300 MHz to about 300 GHz, is used for wireless signals among various devices such as, for example, cellular telephones, personal digital assistants (PDAs), WiFi devices, and navigational systems.
Because many different devices concurrently use the microwave spectrum, government regulations and various agreements have divided it into discrete spectrum bands, which are often further split into smaller sub-bands, thereby minimizing interference. To meet such regulations and agreements, and to meet communication quality requirements, transmitting devices are generally prohibited from emitting energy over a specified level outside of their assigned bands and, preferably, receiving devices are constructed to limit receipt of energy to only their assigned bands
Various microwave filters are therefore incorporated into transmitters and receivers, to limit their broadcast and receipt of signals, respectively, to particular frequencies. For this reason, the performance qualities of the microwave filters often have significant effect on the quality of communications and, further, are a determining factor for spacing between channels and, hence, the usable capacity of the spectrum.
Microwave filters may be configured to have low pass (LPF), band pass (BPF) or high pass (HPF) characteristics, each typically having at least one pass band, transition band and stop band.
For purposes of brevity this disclosure, however, will describe various exemplary embodiments and arrangements in reference to microwave LPFs. This is simply to focus the description on the novel features and aspects of the invention, to better enable persons of ordinary skill in the art to make and use it based on this disclosure. However, otherwise stated or clear from the context, the invention and all of its various embodiments may be readily practiced in alternative arrangements as microwave BPFs and/or HPFs simply by, for example, applying conventional filter design methods to translate or reconfigure the disclosed microwave LPFs to microwave BPFs or HPFs.
As known to persons skilled in the relevant arts, an ideal microwave LPF blocks all frequencies above a given cut-off frequency, has a zero-width transition band, and passes without attenuation all signal frequencies below the cut-off.
Realizable microwave LPFs, however, do not have such characteristics. Realizable microwave LPFs have pass band attenuation, meaning that some of desired signal energy is lost, a finite attenuation, meaning that some undesired signal energy gets through, and a slope-like transition band extending from the cut-off frequency to the reject band. Therefore, among the various measures of microwave LPF transmission quality, three are: stop-band attenuation, band-pass loss, and cut-off slope.
One well-known type of microwave LPF is the stepped-impedance resonator (SIR) filter, which comprises a succession of resonant sections, each section having a high impedance subsection that steps to a low impedance subsection. The resonant sections may be configured in various ways, such as coaxial, microstrip, or strip line.
FIG. 1 is a three-dimensional view of an exemplar coaxial SIR LPF 10 according to the related art, with its outer conductor removed for clarity.
As shown in FIG. 1, a traditional coaxial SIR LPF 10 may comprise a series of N resonator sections, each referenced as 12n, n=1 to N. Each section 12n comprises a low impedance subsection 14n followed by a high impedance subsection 16n which, at microwave frequencies, embody a capacitor and an inductor, respectively. Each section 12n therefore forms an inductor-capacitor (LC) resonator.
FIG. 2 shows a lumped parameter model 20 for a coaxial SIR LPF such as the FIG. 1 exemplar 10.
Referring to FIG. 2, lumped parameter model 20 depicts an SIR LPF such as the FIG. 1 example 10, as comprising N resonator sections 22n, each having an inductor element Ln and a capacitor element Cn, each having a respective reactance value corresponding, in reference to FIG. 1, to the impedance of its modeled subsection 14n and 16n. The relative values of Ln and Cn, each set by physical parameters such as width, length and materials, in turn set the resonant frequency of each set 22n. Therefore, an appropriate LPF characteristic may be obtained by selecting appropriate dimensions and materials for each section 12n.
FIG. 3 shows an illustrative frequency response 30, based on an example seven-pole related art SIR LPF such as, for example, the FIG. 1 exemplar 10. Referring to FIG. 3, the example frequency response 30 has an example upper “cut-off” frequency, labeled 32, at approximately 5 GHz. The 5 GHz value in this example is arbitrary, but the form of the frequency response is representative of a related art seven-pole SIR LPF. The slope of the frequency response 34 above the example 5 GHz cut-off, labeled 32, is not very sharp. This is shown particularly by the attenuation 36 of only approximately 12 dB at approximately 5.5 GHz. Spurious modes may appear at 5.5 GHz, through, due to harmonics, or integral multiples of the resonant sections (not shown in FIG. 3) that form the SIP LPF.
There are known methods directed to solving the problem of spurious bands. All of these methods, however, have shortcomings.
For example, one method is to add another LPF, such as a mask filter, to the SIR LPF. This has drawbacks, though, including increased cost and, particularly, pass-band insertion loss. Further, adding a mask filter in line with a main filter may increase the complexity of the tuning procedure of the overall microwave system.
Another method is the addition of an arrangement of inductors, as described by U.S. Pat. No. 2,641,646 to Thomas. However, the method taught by Thomas may have many of the some of the same shortcomings as using an additional LPF. In addition, Thomas may require the use of heavy wire or copper tubing, materials that may not be appropriate for a low cost microwave LPF microwave cavity.
Another related method directed to solving the problem of spurious modes is taught by Published U.S. Patent Application No. 2003/0001697 to Bennett et al. Bennet teaches intermediate suppression elements, interspersed within the SIR structure. However, this method may require complete reconfiguration of the SIR filter structure